Question

How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?

Answer 1

There are several layers present in the atmosphere. The upper layer is optically rare, but the bottom layer is optically denser. When light passes through thicker and denser layers of the atmosphere, it bends towards the normal. As the stars are present very far away from us, the light coming from them get refracted multiple times and that results in the twinkling of the stars.

The planets are present much closer to the earth as compared to stars which make planets look like an extended source of light, not like a point-sized source of light as stars appear. This makes planets look stationary, not twinkling like stars.